Sometimes people are having rough days. It's not fair for him to take it out on you. But it's pretty expected that he's going to be an asshole and really impatient
I peed in a horse once.
Hey, at least he was nice enough to crumple it up and correct it for you. Some celebs might have just handed it to you misspelled and told you to be on your way.
Personally, I probably would have given him an OK since he fixed it for you. Some wouldn't have bothered.
You got off lightly. I've been to a few cons and some stars are much worse.